Tunable optical pair source and related systems and methods

ABSTRACT

Example embodiments disclose a tunable optical pair source (TOPS) configured to generate first and second output optical beams having respective first and second frequencies that are phase locked with each other. The TOPS may include a first laser, such as a tunable laser, configured to generate a first laser beam, a radio frequency (RF) oscillator configured to transmit an RF reference signal, a beam splitter in optical communication with the first laser, and an electro-optic modulator configured to modulate the second split beam with the RF reference signal to form a modulated beam having a first sideband comb comprising a plurality of harmonics. Additionally, the TOPS may include an optical filter configured to receive the modulated beam and output a filtered optical beam, and a second laser configured to generate a second laser beam at the second frequency, the second laser being configured to receive the filtered optical beam as a seed.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 16/847,417 filed Apr. 13, 2020, which claims priority to U.S.Provisional Patent Application No. 62/833,310 filed Apr. 12, 2019, thedisclosure of each of these application being hereby incorporated byreference in its entirety. This application also incorporates byreference the disclosure titled, “Radiofrequency signal-generationsystem with over seven octaves of continuous tuning,” authored bySchneider et al., and published in Nature Photonics, online Jan. 20,2013, which may provide background information such as exemplary detailsof structure and operation of corresponding elements of thisnon-provisional application.

BACKGROUND

Wireless communications utilize a wide variety of RF frequencies,including S and C bands (2-6 GHz), Ku-band (12-18 GHz), etc. Nextgeneration broadband wireless may utilize at higher RF frequencies.Schneider et al. discloses a photonic radiofrequency system where asingle signal source that can generate carrier waves over a variety ofRF frequency bands, without sacrificing spectral purity. In contrast,purely electrical solutions encounter obstacles to realizing such awidely tunable source, as challenges arise related to high conductionlosses, parasitics and susceptibility to electromagnetic interference(EMI).

FIG. 1 is representative of the prior art system of Schneider et al.Modulation is provided by an electro-optic phase modulator driven by alow-frequency high-purity reference. Phase modulation by a puresinusoidal reference signal yields a comb of sidebands offset from thereference by multiples (harmonics) of the reference. Additionalnonlinear distortion is contributed by including an electronic elementsuch as a saturated amplifier or a nonlinear transmission line, whichyields a broader comb of sidebands. Injecting the entire comb ofsidebands into a clone laser (Laser 2), after filtering to suppress thereference laser's (Laser 1) fundamental line, phase-locks the Laser 2with Laser 1.

Wide tunability is realized by injection locking using a broad comb ofharmonics, all derived from externally modulating reference Laser 1 witha low-frequency RF reference that has been subject to nonlineardistortion. The output of Laser 1 may be used as the optical carrier.Laser 2 is a clone laser that is tuned to match and lock to thefrequency of any one of the injected harmonics and is used as areference beam. When the outputs of Laser 1 and Laser 2 are combined, acombined optical signal is generated having a beat frequency equal tothe offset (difference) between the frequencies of Laser 1 and Laser 2.Choosing higher harmonics allows very high offset frequencies to beobtained, and because the locked lasers have identical phase noise, thepurity of the reference is preserved.

In the prior art system of FIG. 1, the primary laser (Laser 1) is fixed.Tunability of the system is achieved by generating and injecting a widecomb of sidebands into the clone laser (Laser 2) that are offset fromLaser 1 (the primary laser) by harmonic multiples of a low-frequencytunable RF reference oscillator, and tuning the clone laser (Laser 2) toselect one of the harmonics of the injected comb of sidebands.Specifically, prior to injection into the clone laser (Laser 2), theoptical carrier is suppressed by an optical filter to reject the opticalcarrier while passing one entire sideband comb; or interferometrically,as by null biasing in a Mach-Zehnder Interferometer (MZI) type modulator(when the optical carrier is suppressed in a Mach-Zehnder Interferometer(MZI) type modulator, even harmonics are also suppressed along with thecarrier, and both upper and lower sideband combs comprising oddharmonics are injected into the clone laser (Laser 2)). The resonance ofthe injected laser cavity of the clone laser (Laser 2) is thermallytuned to match a targeted and/or selected harmonic in the sideband combinjected into the clone laser (Laser 2). This technical setup leads thelaser beam of the clone laser (Laser 2) to lock to that particularselected harmonic of several harmonics that are injected into the clonelaser (Laser 2).

As noted, in the prior art system of FIG. 1, harmonics other than theselected harmonic in the comb are also injected into the clone laser(Laser 2). Although these non-selected harmonics are not resonantlyamplified in the laser cavity of the clone laser (Laser 2), they arepresent in the output of the clone laser (Laser 2) as spurs. As aresult, when the output of the clone laser (Laser 2) is combined withthe reference laser (Laser 1), the non-desired harmonics beat inconjunction with the output of the reference laser (Laser 1) output onthe photodiode detector which leads to a particular problem ofgenerating undesired spurious RF output harmonics.

SUMMARY

Exemplary embodiments may disclose a tunable optical pair source (TOPS)configured to generate first and second output optical beams havingrespective first and second frequencies that are phase locked with eachother. The TOPS may include, a tunable laser configured to generate afirst laser beam at the first frequency, the first frequency beingvariable, and a radio frequency (RF) oscillator configured to transmitan RF reference signal oscillating at a radio frequency. The TOPS mayfurther include, a beam splitter in optical communication with thetunable laser and configured to split the first laser beam and transmitcorresponding first and second split beams, a first optical path inoptical communication with the beam splitter to receive the first splitbeam and to provide the first output optical beam, and a second opticalpath in optical communication with the beam splitter to receive thesecond split beam and to provide the second output optical beam. Thesecond optical path may include, an electro-optic modulator configuredto modulate the second split beam with the RF reference signal to form amodulated beam having a first sideband comb comprising a plurality ofharmonics, an optical filter configured to receive the modulated beamand output a filtered optical beam, and a fixed laser configured togenerate a second laser beam at the second frequency, the fixed laserbeing configured to receive the filtered optical beam as a seed. In thedisclosed TOPS, the first frequency and the second frequency may beoffset by an integer multiple of the RF frequency, and the secondfrequency may fall within the passband of the optical filter.

In some embodiments, the TOPS may further include an optical circulatorhaving first, second, and third ports. The optical circulator may beconfigured to receive the filtered optical beam at the first port andpass the filtered optical beam via the second port to the fixed laser,and receive the second laser beam at the second port and pass the secondlaser beam via the third port to an output of the TOPS. In someembodiments, a TOPS system, such as an antenna (e.g., phased arrayantenna) may be formed of a TOPS, a combiner that combines the outputsof the TOPS into a combined beam, a third optical path extending fromthe combiner to a photodetector that is configured to detect thecombined beam.

Exemplary embodiments may disclose a method of generating first andsecond output optical beams having respective first and secondfrequencies that are phase locked with each other. Exemplary methods mayinclude generating a first laser beam at the first frequency, the firstfrequency being variable, and transmitting an RF reference signaloscillating at a radio frequency. Exemplary methods may further include,splitting the first laser beam and transmitting corresponding first andsecond split beams, and receiving the first split beam and providing thefirst output optical beam in a first optical path. Exemplary methods mayfurther include, receiving the second split beam and providing thesecond output optical beam in a second optical path, and modulating thesecond split beam with the RF reference signal to form a modulated beamhaving a first sideband comb comprising a plurality of harmonics.Exemplary methods may further include, receiving the modulated beam atan optical filter and outputting a filtered optical beam, and receivingthe filtered optical beam as a seed and generating a second laser beamat the second frequency. In exemplary methods, the first frequency andthe second frequency may be offset by an integer multiple of the RFfrequency, and the second frequency may fall within a passband of theoptical filter. In some exemplary methods, combining the first opticalbeam and the second optical beam to thereby form a combined beam, anddetecting the combined beam may additionally be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure will now be described more fully with referenceto the accompanying drawings, in which various embodiments are shown.The invention may, however, be embodied in many different forms andshould not be construed as limited to the exemplary embodiments. Likenumbers refer to like elements throughout the drawings, which includethe following:

FIG. 1 is an illustration of a prior art device;

FIG. 2 is a schematic diagram of an exemplary embodiment;

FIG. 3 is an exemplary method; and

FIG. 4 is an exemplary graph illustrating some aspects of the improvedresults of the exemplary embodiment of FIG. 2.

DETAILED DESCRIPTION OF THE EMBODIMENTS

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the disclosure. As usedherein, the term “and/or” includes any and all combinations of one ormore of the associated listed items.

It will be understood that when an element is referred to as being“connected” or “coupled” to another element, it can be directlyconnected or coupled to the other element or intervening elements may bepresent. In contrast, when an element is referred to as being “directlyconnected” or “directly coupled” to another element, there are nointervening elements present. Other words used to describe therelationship between elements should be interpreted in a like fashion(e.g., “between” versus “directly between,” “adjacent” versus “directlyadjacent,” etc.).

The terminology used herein is for the purpose of describing particularembodiments and is not intended to be limiting of the inventive conceptdisclosure and claims. As used herein, the singular forms “a,” “an” and“the” are intended to include the plural forms as well, unless thecontext clearly indicates otherwise. It will be further understood thatthe terms “comprises,” “comprising,” “includes” and/or “including,” whenused herein, specify the presence of stated features, integers, steps,operations, elements, and/or components, but do not preclude thepresence or addition of one or more other.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art of this disclosure. It will be furtherunderstood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein.

FIG. 2 illustrates an improved system which may address one or more ofthe technical problems discussed above with respect to prior artsystems. A notable difference between the prior art systems and thepresent disclosure is that the primary laser (or reference laser) 201 istunable across a range of frequencies. The secondary laser (or clonelaser) which is injected with a seed resulting from modulation of thelaser beam output by the tunable laser 201 may be implemented as a fixedlaser 215 (although a tunable laser may also be used here). Selection ofa harmonic for seed injection to the secondary laser 215 may beperformed without requiring tuning of the secondary laser 215. Forexample, selection of the harmonic (and frequency of the secondary laserbeam) may be performed by use of a narrow pass-band optical filter 213having a passband sufficient to encompass only a single harmonicsideband from the input sideband comb (and exclude or suppress others).The optical filter passband is matched to the frequency of the fixedlaser 215.

When including an optical filter 213 that has a very narrow passband,the TOPS may efficiently attenuate unwanted harmonics and leave only oneharmonic at significant enough signal strength for injection seeding tothe fixed laser 215. The secondary laser thus may automatically lock tothe harmonic selected by the optical filter without requiring tuning toselect between one of many harmonics. Reducing the number of injectedharmonics improves the operation of the fixed laser 215 as it locksbetter to the selected injected wavelength, which reduces instabilitiesand phase noise. Similarly, an optical filter 213 that is fixed hasgenerally better filtering characteristics (is more selective) ascompared to a conventional tunable filter. However, it should beunderstood that tunable optical filters may certainly be used in placeof a fixed filter. However, a fixed filter may have certain advantagesas, generally, it (1) reduces the complexity of the system and (2)provides improved performance compared to a tunable filter. However,those with skill in the art will appreciate that having a tunability ofthe optical filter may be desired in some configurations. For example, atunable optical filter may be desired to allow slightly adjusting thecenter of the passband to better match the frequency to the freeoscillations of the secondary laser (e.g., fixed laser 215) while stillhaving the bulk of the advantages of a fixed filter (e.g., by virtue ofonly being limited in its tunability over a narrow range).

FIG. 2 illustrates and exemplary embodiment of the present invention. Atunable optical pair source (TOPS) 200 is configured to generate andoutput at outputs 220 a and 220 b first and second output optical beamshaving respective first and second frequencies (ω₁ and ω₂) that arephase locked with each other is shown. The TOPS 200 includes a tunablelaser 201 and a fixed laser 215. The tunable laser 201 is configured togenerate a first laser beam at the first frequency ω₁ (represented byspectrum 2 a in FIG. 2) where the first frequency ω₁ is adjustable. Forexample, the tunable laser 201 may be thermally tuned or adjusted by auser from an external control to emit the first laser beam of a selectedand/or adjustable frequency. Additionally, after selection of the firstfrequency ω₁, the tunable laser 201 may operate to generate the firstlaser beam at the selected first frequency ω₁ in a substantiallyconstant manner (i.e., so that the frequency of the first laser beamremains substantially constant, such as with acceptable variations infrequency due to noise). In later, a subsequent operation, the tunablelaser 201 may have the first frequency oil adjusted to a frequencydifferent from its previous operation. Additionally, the tunable laser201 may be tunable across the RF frequency range of the RF signals inputinto EO modulator 211. Thus, the frequency of the tunable laser may beadjusted in the higher frequency optical domain over a rangecorresponding to this RF frequency range. In some embodiments, a firstfrequency of the tunable laser 201 may be adjustable at least by anamount corresponding to a bandwidth of a first sideband comb (see FIG.2).

In the TOPS 200, the first laser beam generated by tunable laser 201 istransmitted to a beam splitter 205. The beam splitter 205 may be inoptical communication with the tunable laser by any known means, e.g.,by an optical waveguide such as an optical fiber. The beam splitter 205is configured to split the first laser beam originating from the tunablelaser 201 to generate corresponding first and second split beams (eachhaving the first frequency ω₁). The first split beam is transmitted viaa first optical path 207 to output 220 a of the TOPS 200. The firstoptical path 207 may be formed by an optical waveguide, such as anoptical fiber. Although not shown in FIG. 2, other optical elements maybe provided within first optical path 207 (e.g., beam splitter,combiner, etc.) The second split beam provided by beam splitter 205 maybe transmitted to an electro-optic modulator 211 via a second opticalpath 209. In this example, the second optical path 209 provides anoptical communication path from beam splitter 205 to output 220 b of theTOPS 200. The output 220 b of the TOPS 200 is in optical communicationwith beam splitter 205 via the second optical path 209 and provides asecond output optical beam of frequency ω₂ that is both phase lockedwith the first output optical beam and has a frequency ω₂ that is offsetfrom the first frequency ω₁ by a set value. In this example, the secondoptical path 209 provides optical communication from the beam splitter205 to output 220 b and extends through the following elements in thefollowing sequence: from beam splitter 205, to electro-optic modulator211, to optical filter 213, to optical circulator 217, to fixed laser215, back to the optical circulator 217 and to output 220 b of the TOPS200. In addition, the second optical path 209 includes opticalwaveguides, such as optical fiber, connecting these optical elements toprovide the various optical signals therebetween. It will be appreciatedthat the single lines extending between the elements shown in FIG. 2represent optical waveguides (with arrow heads denoting a direction oftransmission), while the double lines extending between the elementsshown in FIG. 2 represent RF waveguides (e.g., coaxial cable, striplines, microstrips, twisted pair, etc.) or other electrical wiring.

The TOPS 200 may be provided with a comb of harmonics provided from aradio frequency (RF) oscillator 203 and amplifier 202. RF oscillator 203is configured to generate an electrical RF reference signal oscillatingat a radio frequency. For example, the oscillator 203 may generate an RFreference signal having a frequency within the range of several kHz totens of GHz, such as between 30 kHz to 100 GHz or more. The RFoscillator 203 may be a voltage controlled oscillator and have a voltageinput to adjust the frequency of the RF reference signal generated bythe RF oscillator 203. The frequency of the RF reference signalgenerated by the RF reference oscillator may be altered, such as beingselectable by a user (e.g., via programming) or otherwise automaticallydetermined.

The RF reference signal generated by the RF oscillator 203 is providedto an amplifier 202. The amplifier 202 may be a saturated RF amplifierand generate a comb of RF harmonics from the RF reference signalprovided by the RF reference oscillator 203. The comb of RF harmonicsmay comprise an electrical signal having plurality of harmonics (or RFtones) at frequencies equal to a corresponding integer multiple of theRF reference signal provided by the RF oscillator 203. For example, whenthe RF reference signal provided by RF reference oscillator 203 has afrequency of Ω, the comb of RF harmonics may have RF harmonics (tones)at frequencies of Ω, 2*Ω, 3*Ω, n*Ω (where n is a positive integer).Electronic circuitry other than an amplifier 202 may be used to generatethe comb of RF harmonics, such as other electronic components, such as anonlinear transmission line, or other electronic circuitry comprising anonlinear element.

The comb of RF harmonics generated by the amplifier 202 is provided toan electro-optic modulator 211, e.g., a high-speed electro-optic (EO)modulator, or a Mach-Zehnder-Interferometer (MZI) modulator. Theelectro-optic modulator 211 modulates the second split beam (having afrequency of ω₁) provided by beam splitter 205 with the comb of RFharmonics to generate a modulated beam comprising an optical signalhaving a center frequency of ω₁ and first and second sideband combs of aplurality of harmonics (as represented in spectrum 2 b of FIG. 2). Eachsideband comb of this modulated beam corresponds to the comb of RFharmonics provided to the electro-optic modulator 211 as upconverted toan optical signal. Thus, the frequencies of the harmonics of themodulated beam output by the electro-optic modulator 211 are offset fromω₁ by a corresponding integer multiple of Ω, (or an integer multiple ofthe frequency of the RF reference signal generated by RF oscillator203). Spectrum 2 b of FIG. 2 shows teeth of the sideband combs of themodulated beam representing corresponding harmonics in the optical realmat locations of ω₁+/−n*Ω (where n is an integer). It should beappreciated that generation of a comb of harmonics (at both the RFfrequencies and corresponding optical frequencies) is not necessary. Forexample, the RF reference signal provided by the RF reference oscillator203 may be provided directly to the electro-optic modulator 211 as asingle RF harmonic resulting in sidebands of the modulating beam eachhaving a single harmonic. It should be understood, unless contextindicates otherwise, that the description of various systems and methodsherein also applies to such single harmonic implementation.

The modulated beam generated by electro-optic modulator 211 istransmitted along the second optical path 209 to an optical filter 213.The optical filter 213 receives and filters the modulated beam andoutputs a filtered optical beam along the second optical path 209. Thedashed line in spectrum 2 b illustrates an exemplary passband(encompassing frequency ω₂) of the optical filter 213, which whenapplied to the spectrum 2 b via optical filter 213 results in exemplaryspectrum 2 c of FIG. 2 (which represents the output of optical filter213). The optical filter 213 may have a very narrow passband thatincludes and/or is centered upon the operational frequency of fixedlaser 215. The optical filter may have passband width of less than 2*Ωand to encompass only a single harmonic of the modulated beam (withother harmonics outside the passband) and thereby suppress and/oreliminate the remaining harmonics of the modulated beam. For example,the RF oscillator 203 may be configured to generate an RF referencesignal of a frequency (a) 2.4 GHz, (b) 5 GHz, (c) 28 GHz, (d) 37 GHz,(e) 39 GHz and/or with RF frequencies in the ranges of (f) 27.5-28.35GHz, (g) 37-40 GHz and (h) 64-71 GHz. For cases (a) to (h), the opticalfilter 213 may have a passband width of (a) less than 4.8 GHz, (b) lessthan 10 GHz, (c) less than 56 GHz, (d) less than 74 GHz, (e) less than78 GHz (f) less than 55 GHz, (g) less than 74 GHz and (h) less than 128GHz, respectively. For example, the optical filter 213 may have apassband of less than 6 GHz, such as less than 5 GHz. Additionally, insome embodiments the suppression of the remaining harmonics (other thanthe passed single harmonic) of the modulated beam may be attenuated byat least 3 dB. For example, the optical filter 213 may have a passbandof 5 GHz or less (where the outer range of the passband corresponds to a3 dB attenuation as compared to maximum power transmission) where out ofband suppression is at least 20 dB, such as at least 30 dB (where suchsuppression is reached at an offset of approximately 10 GHz from thecenter of the filter passband). When the passband of the optical filter213 encompasses only a single harmonic, the optical filter 213 may beresponsive to identify (select) the harmonic to be resonated by laser215, thereby not necessitating laser 215 to filter and select betweenseveral harmonics as the resonant frequency of laser 215. As the laser215 may be relieved of this filtering role, laser 215 may be implementedto provide a wider locking range, which may improve the overallphase-noise performance of the system.

As described herein, the filter 213 may select a different one of theharmonics of the modulated beam by adjusting the frequency of thetunable laser 201, thus shifting the center frequency (ω₁) of themodulated beam along with the sideband combs (and thus shifting each ofthe harmonics of the sideband combs). Also, the frequency Ω of the RFreference oscillator 203 may be adjusted to maintain a particular toothof the comb in the passband of filter 213 as the center frequency ω₁ oftunable laser 201 is shifted. Additionally, in some embodiments, a widthof the passband of the filter 213 may be greater than 2*Ω but less thana bandwidth of the first sideband comb of the modulated beam.

Use of a fixed optical filter for optical filter 213 may be preferred incertain implementations as a fixed optical filter may provide betterfiltering characteristics (e.g., be more selective) than a tunableoptical filter. A fixed optical filter may also reduce the complexity ofthe TOPS providing a more economic and robust configuration. The filter213 may be implemented as a Fiber Bragg Grating filter, a dielectricstack filter, an interferometric filter, such as a Fabry-Perotinterferometer (or etalon) or Mach-Zehnder interferometer (or etalon)with a differential path length, an optical ring resonator (or ringcavity), an optical filter having a photonic-crystal. In some examples aconventional DWDM filter/multiplexer may be used. Filter 213 may beimplemented as one of these optical filters or several of these opticalfilters cascaded together. Alternatively, the optical filter 213 may betunable. In some examples, the tunability may be less than +/−Ω or lessthan +/−0.5*Ω to align (shift closer to each other) the center of thepassband with a center of the selected harmonic (within the passband) ofthe modulated beam. Tunability may be implemented by controlling thesize of the optical filter 213, such as by expanding or contracting theoptical filter 213 by controlling its temperature and/or applying acompressive or tensile force to the optical filter (e.g., via apiezoelectric transducer), or by changing the angle and/or shifting theposition or orientation of a dielectric-stack filter.

The TOPS may include an optical circulator 217. In some embodiments, theoptical circulator 217 may have first, second, and third ports. In theexample of FIG. 2, a first port receives the filtered optical beam fromthe optical filter 213 and the second port transmits/passes the filteredoptical beam to the fixed laser 215. The second port also receives thesecond laser beam from the fixed laser 215 and the third porttransmits/passes the second laser beam of the fixed laser 215 to thesecond output 220 b of the TOPS.

The TOPS may include a fixed laser 215 that is configured to generate asecond laser beam at a second frequency (ω₂) (as represented by spectrum2 d in FIG. 2). The second frequency (ω₂) may fall within the passbandof the optical filter 213. The fixed laser 215 may be an injectionlocked laser and produce an injection locked, or phase-locked, lasersignal. The fixed laser 215 may be considered a clone laser with tunablelaser 201 being considered the reference laser. The fixed laser 215receives the filtered optical beam from optical filter 213 via opticalcirculator 217 and uses the same as a seed. For example, the fixed laser215 may be seeded with the filtered optical beam.

In other examples, the fixed laser 215 may receive the filtered opticalbeam without use of an intervening circulator, such as (for example), asbeing directly coupled with rom the optical filter 213 with an opticalwaveguide, without other intervening optical components interpose onthat portion of the second optical path. In this alternativeimplementation without circulator 217, a separate port into thesecondary laser 215 may be used to receive and input the filteredoptical beam for injection locking. Specifically, the laser cavity offixed laser 215 may be structured so as to provide for coupling of lightto and from the laser cavity at two separate ports. For example, aFabry-Perot cavity may be used where a semi-transparent mirror is usedat both ends of the cavity so that each end of the cavity may serve as aport. In this configuration, filtered optical beam from optical filter213 may (the seed signal) may be injected through a first port and thesecond laser beam may be output from a second port of the laser cavityof fixed laser 215. Such a configuration may not need circulator 217 toseparate the injected signal from the second laser beam output fromfixed laser 215.

As represented in spectrum 2 d, filtered optical beam received by thefixed laser 215 (represented by dashed lines) may include attenuatedharmonics (attenuated by the optical filter 213 due to falling outsideits passband) on either side of the harmonic selected by the opticalfilter 213 (at frequency ω₂). These attenuated harmonics may be furtherfiltered (attenuated), relative to the selected harmonic, as a result ofthe fixed laser 215 automatically locking on to the selected harmonic(at frequency ω₂) in generating the second laser beam (at frequency ω₂).The first frequency ω₁ of the tunable laser 201 and the second frequencyω₂ of the fixed laser 215 are offset from one another by an integermultiple of the RF frequency Ω.

Additionally, the tunable laser 201 may be tuned to shift a desiredharmonic of the modulated beam to a frequency range within the passbandof filter 213. Tuning laser 201 in this way may be used to select thedesired integer multiple of the RF frequency Ω by which the frequenciesof the first and second output optical beams are offset. It will beapparent that tuning laser 201 to shift the first frequency ω₁ of thetunable laser 201 causes a shift in the frequency (also ω₁) of the firstoutput optical beam as well as a shift of the sideband combs of themodulated signal (i.e., all of the spectrum 2 b is shifted). However,when the optical filter remains substantially fixed (e.g., the dashedline in spectrum 2 b representing optical filter 213 passband is notshifted), a new harmonic (a new sideband tooth) of the modulated signalis eventually shifted into the passband of optical filter 213 to beselected by optical filter 213. Thus, the frequency ω₂ of the secondlaser beam generated by fixed laser 215 may correspond to the frequencyof a different harmonic of the modulated signal (but remaining in therange of the passband of the optical filter 213). As each harmonic ofthe modulated signal is offset from the center frequency ω₁ of themodulated signal by a corresponding integer multiple of the RF referencefrequency of Ω (the frequency of each harmonic=ω₁+/−n*Ω), tuning thetunable laser 201 in this way may be used to select this integermultiple of the RF reference frequency of Ω (i.e., select the value ofthe integer “n” in ω₂=ω₁+/−n*Ω), The resonant operational frequency ofthe fixed laser 215 may be aligned with the center frequency of thepassband of the optical filter 213 or otherwise match the optical filter213 to fall within the passband of the optical filter 213.

It should be appreciated that the frequency of the second laser beam oflaser 215 may vary (to align with the frequency of the selectedharmonic) even when the second laser beam is generated by a fixed laserand not generated by a tunable laser. As used herein, a tunable laser201 may refer to a laser designed so that the wavelength of light itgenerates may be changed, or tuned, over a certain range. In the case ofsemiconductor lasers, the tuning may be achieved by a controller (e.g.,computer) that controls a change in the temperature at which the laseroperates via the use of a thermo-electric cooler or a heater, and/orthat controls a change in the injection current of the laser. Whendesigning a system with a tunable laser, e.g., tunable laser 201,provisions may be made for varying the temperature or injection current,or both. In contrast, for a fixed laser, e.g., fixed laser 215, care maybe taken in designing the circuitry powering it and environmentalcontrol to keep them stable so as to avoid unintentional change in theoperating wavelength. For example, a fixed laser may have a controlsystem (e.g., a controller, such as a computer) to maintain asubstantially constant injection current and/or substantially constanttemperature of the fixed laser. Both the fixed laser 215 and the tunablelaser 201 may be implemented using similar, or identical, semiconductorchips. Power control circuitry and environment control circuitrysurrounding the chip or chips, in which the resonant cavity of laser isformed determines whether such a laser constitutes a fixed laser ortunable laser. Generally, a fixed laser, e.g., laser 215 may include astable power supply and a stable environment whereas for a tunablelaser, e.g., tunable laser 201 may include a power supply and/orenvironment that are controlled to be altered. Thus, it should beappreciated that a fixed laser may have frequency that is not perfectlyfixed. A fixed laser has a finite linewidth, indicating how much itsfrequency tends to vary even when there is no intentional variation ofits inputs/operating conditions. Thus, laser 215 may be considered fixedby virtue of the absence of power control circuitry and environmentalcontrol circuitry configured to alter the operating frequency from afirst target (i.e., selectable) operating frequency to a second targetoperating frequency in response to some direction (e.g. user inputand/or programming) of the overall system operation.

In some embodiments, the fixed laser 215 may be substantially fixedduring operation. However, the fixed laser 215 is phased locked with aharmonic offset from the frequency of the laser beam of the tunablelaser 201. Thus, changes to first laser beam of the tunable laser 201,whether intentional or unintentional (i.e., noise), whether in phase orfrequency, are correspondingly reproduced in the second laser beam ofthe fixed laser 215.

The TOPS 200 may be implemented in a variety of TOPS systems, such as avariety of wireless communication systems. FIG. 2 also illustrates anexemplary TOPS system, including the TOPS 200, a combiner 302, aphotodetector 304, an antenna 310 and one or more electro-opticmodulators 312 that may be used with the TOPS 200. U.S. patentapplication Ser. No. 16/198,652, filed Nov. 21, 2018, (herebyincorporated by reference in its entirety) provides exemplary details ofthe structure and operation of the elements of such a TOPS system, aswell as exemplary details of the structure and operation of other TOPSsystems that may be implemented with the improved TOPS of the presentinvention. For example, modulation of the one or more electro-opticmodulators 312 may be used to encode information to provide the same aspart of the combined optical beam (e.g., by phase and/or amplitudemodulation of one or more of the first and second output optical beams)according to a variety of encoding schemes, such as OFDM, QAM, etc. Theencoded information may be represented in corresponding phase and/oramplitude modulation of the beat RF signal and the RF electromagneticwave output by antenna 310. The combiner 302 may be disposed where thefirst optical path 207 and the second optical path 209 converge, and maycombine the output of the first optical path 207 (the first opticalbeam) and the output of the second optical path 209 (the second opticalbeam) to form a combined optical beam. The combined optical beam istransmitted via a third optical path 306 to photodetector 304. The thirdoptical path may be embodied by an optical waveguide, such as an opticalfiber. Spectrum 2 e is representative of the combined optical beam,showing the combined optical beam comprising frequency components of ω₁and ω₂ (corresponding to the contributions of the first and secondoutput optical beams forming the combined optical beam).

One or more modulators 312 may be provided to phase and/or amplitudemodulate one or both of the first and second output optical beams outputby TOPS 200. FIG. 2 illustrates options for providing a modulator 312 ain first optical path 207 to modulate the first output optical beamand/or a modulator 312 b in the second optical path 209 to modulate thesecond output optical beam (prior to their being combined by combiner302). After phase and/or amplitude modulating one or both of the firstand second output optical beams with modulators 312 a and/or 312 b, thefirst and second optical beams may be combined by combiner 302 tointerfere with each other in the combined optical beam.

Alternatively, modulation of a modulator 312 c may be provided toreceive the combined optical beam from combiner 302. In thisconfiguration, the combiner 302 may combine the first and second outputoptical beams so that their polarization directions (linear or circular)are orthogonal to each other in the combined optical beam. Modulator 312c may be a vector modulator and phase and/or amplitude modulate one orboth of the first and second output optical beams and alter thepolarization direction of one or both of the first and second outputoptical beams so that they interfere with one another.

When the polarization direction of the combined first and second opticalbeams forming the combined optical beam are (at least partially) alignedand not orthogonal to each other (as provided by either the combiner 302or modulator 312 c), the first and second output optical beams mayconstructively and destructively interfere with each other. The combinedoptical beam may thus have a beat frequency of |ω₂−ω₁|, an RF frequencycorresponding to an integer multiple of the RF reference frequency Ω(i.e., n*Ω). Further, any modulation of the first and/or second outputoptical beams also is reflected in modulation of this RF beat signal.This modulation of the beat signal corresponds to the modulation, bothin amplitude and phase, of the RF electromagnetic wave output by thecorresponding antenna 310 (discussed below).

The photodetector 304 receives the combined optical beam to generate anRF electrical signal (a photocurrent) corresponding to the RF beatfrequency of the combined optical beam. The photocurrent produced byphotodetector 304 may faithfully regenerates an original input RF signalto the TOPS. The intrinsic noise components of the phases of lasers 201,215 correlate to each other and may be identical or otherwise correlateto each other, and hence may not contribute to the time-varyingcomponent of the photocurrent, but may rather cancel out.

The RF electrical signal output of photodetector 304 may then drive orcontrol antenna 310 to be transmitted as an electromagnetic wave. The RFelectrical signal may be applied directly to antenna 310 (without an RFtransmission line or an amplifier), or may be transmitted to antenna 310via an RF transmission line 308 with optional amplification by anamplifier inserted in the RF path from the photodetector 304 to antenna310 (amplifier not shown). In some examples, the RF signal output by thephotodetector may be processed by a computer for data extraction and/orsynthetization (e.g., after downconversion to baseband and digitizationby an A/D converter). Owing to the superposition principle obeyed by thetotal electric field incident on the photodetector 304, the photocurrent(and hence the output RF signal) may be dependent on the phasedifference of lasers' 201, 215 outputs and, consequently, the frequencyof the photocurrent's oscillations proportional to (e.g., an integermultiple of) the RF-reference-signal frequency of the RF oscillator 203.

Additionally, the combined optical beam may have properties and/orattributes where the phase noise of the first optical beam and the phasenoise of the second optical beam are correlated (see FIG. 2). The phasenoise may be correlated as a result of the particular configurationsdescribed hereinabove and illustrated in FIG. 2. Consequently, opticalcontributions to phase noise in the RF output from the tunable laser 201and from the fixed laser 215 are suppressed.

The TOPS system of FIG. 2 may also include a controller 400. Thecontroller, like other controllers described herein, may be a computer,such as a general purpose computer or may be dedicated hardware orfirmware (such as, for example, a digital signal processor (DSP) or afield-programmable gate array (FPGA)). The computer may be configuredform several interconnected computers. The computer may be configured bysoftware to perform/effect the relevant methods, functions and/oroperations described herein. The controller 400 may provide controlinputs to tunable laser 201 and RF oscillator 203 in a coordinatedmanner. The control inputs of controller 400 may be in the form ofcommands or may directly control the operation of tunable laser 201 andRF oscillator 203. The controller 400 may receive an input 402 which mayselect the RF carrier frequency to be generated by the TOPS system(where the RF carrier frequency may be the RF beat frequency of thecombined optical signal described herein to control operation of antenna310). Input 402 may be a user input which may be in the form of aconfiguration of switches, a configuration of a fuse set, a programmedEEPROM, a software program, etc.

For example, a desired frequency of the second laser beam may be ω₂(e.g., in this example, the desired frequency ω₂ corresponding to apredetermined value correlating to the center of or otherwise within thepassband of optical filter 213 (and corresponding to the operationalfrequency range of second fixed laser 215)). As described herein, toobtain the desired second laser beam frequency ω₂ at a selected Nthharmonic to provide a desired RF beat frequency (and corresponding RFcarrier frequency of the antenna 310) of N*Ω (where N*Ω=|ω₂−ω₁|), thecontroller 400 may control (e.g., provide a command to) tunable laser201 to generate an initial first laser beam frequency ω₁ and control(e.g., provide a command to) reference oscillator 203 to generate the RFreference signal with an RF frequency of Ω. As noted herein, by virtueof the passband of optical filter 213 encompassing the Nth harmonic, theNth harmonic of the sideband is passed to the fixed laser 215 to seedfixed laser to generate the second laser beam at a frequency of ω₂. Asdescribed herein, the Nth harmonic has a frequency of ω₂=ω₁+/−N*Ω, thusobtaining the desired RF beat frequency (and RF carrier frequency ofantenna 310) of N*Ω.

During a later operation, the desired RF beat frequency (and RF carrierfrequency of antenna 310) may be altered by coordinating the control ofthe tuning of laser 201 and the RF frequency Ω of the reference signalof RF oscillator 203. Such a later operation may be a real-timealteration of the TOPS system to alter in real-time the RF beatfrequency and RF carrier frequency of antenna 310, or the lateroperation may be a reconfiguration of the TOPS system for a differentlater implementation (e.g., with a different antenna or antenna array tooperate at a different RF carrier frequency). For example, it may bedesired to change the RF beat frequency of |ω₂−ω₁₋₁| in an initialconfiguration of the TOPS system to an RF beat frequency of |ω₂−ω₁₋₂| ina subsequent configuration. Assume an initial configuration of an RFbeat frequency of ω₁₋₁, with an RF frequency of RF oscillator 203 of Ω₁via selection of an Nth harmonic is obtained in a manner as describedabove. To achieve the desired subsequent configuration (with asubsequent RF beat frequency and RF carrier frequency of |ω₂−ω₁₋₂|), thecontroller 400 may controller may control (e.g., provide a command)tunable laser 201 to alter the frequency of the generated a first laserbeam to ω₁₋₂ while at the same time control RF oscillator 203 to alterthe RF frequency of the RF reference signal to Ω₂. The frequency of thesecond laser beam may be maintained at ω₂ or otherwise maintained withinthe passband of the optical filter 213 in both the first configurationand the second configuration of the TOPS system. As noted herein, insome embodiments, when the frequency of ω₁ of the first laser beam ischanged, the frequencies of comb of sidebands may change correspondinglyand the optical filter 213 may select a new harmonic to pass to thefixed laser 215 (e.g., change selection of Nth harmonic in the firstconfiguration to the (N+1)th harmonic in the second configuration).However, in this example, controller 400 may control the RF frequency Ωto be altered by 1/N*Δω₁ as a function of the change of ω₁ (thefrequency of the first laser) (where N is the integer number of theharmonic falling within with optical filter 213). Specifically, when ω₁is increased, the RF frequency Ω may be decreased by 1/N*Δω₁ and when ω₁is decreased, the RF frequency Ω may be increased by 1/N*Δω₁ thusmaintaining the Nth harmonic at ω₂ and within the passband of theoptical filter 213, while changing (i.e., tuning) the frequency of thefirst laser beam of tunable laser 201 and thus the RF beat frequency ofthe combined beam and RF carrier frequency of antenna 310. In thisexample, changing the frequency of the first laser from ω₁₋₁ to ω₁₋₂ maybe made while altering RF frequency of the RF reference signal from Ω₁to Ω₂ where Ω₂=Ω₁−1/N*|ω₁₋₂−ω₁₋₂| (when ω₁₋₂>ω₁₋₁) orΩ₂=Ω₁+1/N*|ω₁₋₂−ω₁₋₂| (when ω₁₋₂<ω₁₋₁) and N is the Nth harmonic that iswithin the passband of optical filer 213. Although the above example hasbeen made with respect to first and second configurations withrespective first and second sets of frequencies, it will be appreciatedthat such coordinated alteration of the configuration of the TOPS systemmay be continuous and/or provided performed with respect to any numberof configurations. However, if the RF beat frequency/RF carrierfrequency are altered to a significant extent, it may be beneficial toalso modify the antenna 310 (e.g., replace with a new antenna) toprovide an operating frequency of the antenna 310 corresponding to theRF beat frequency.

To maintain the frequency of the second laser beam at ω₂ (e.g., tomaintain at the center of or within the passband of the optical filter213) in both the first and second configurations, controller 400 maycontrol the frequency of the first laser beam to be is the frequency ofthe second laser beam of fixed laser 215, ω₁₋₁ is an initial frequencyof the first laser beam of laser 201, and ω₁₋₂, is an adjusted frequencyof the first laser beam. To maintain ω₂ is the frequency of the secondlaser beam

FIG. 3 is an exemplary method in accordance with the disclosure. Thesteps as illustrated in FIG. 3 may be implemented in conjunction withthe TOPS 200 as disclosed herein with respect to FIG. 2. As illustratedin FIG. 3, a method of generating first and second output optical beamshaving respective first and second frequencies that are phase lockedwith each other is disclosed. Additionally, it shall be understood thatcertain steps disclosed here below may be performed simultaneouslyand/or continuously, although they will be explained sequentially forease of explanation. Step 251 may include: generating a first laser beamat a first frequency, and the first frequency may be variable, e.g., bytunable laser 201. Step 253 may include: generating an RF referencesignal oscillating at a radio frequency, e.g., by reference oscillator203. Step 255 may include: splitting the first laser beam andtransmitting corresponding first and second split beams, e.g., by beamsplitter 205. Step 257 may include: transmitting the first split beamvia a first optical path, e.g., first optical path 207. Step 259 mayinclude: transmitting the second split beam via a second optical path,e.g. second optical path 209. Step 261 may include: modulating thesecond split beam with the RF reference signal or a comb of harmonicsgenerated from the RF reference signal to form a modulated beam havingfirst and second sideband combs comprising a plurality of harmonics,e.g., by modulator 211. Step 263 may include: filtering the modulatedbeam with an optical filter and outputting a filtered optical beam,e.g., by optical filter 213 which may be a fixed filter. Step 265 mayinclude: providing the filtered optical beam as a seed to a fixed laserand generating a second laser beam at the second frequency, e.g., byfixed laser 215. The first frequency and the second frequency may beoffset by an integer multiple of the RF frequency of the RF electricalsignal, and the second frequency falls within the passband of theoptical filter. The first split beam and the second laser beam may beoutput by the TOPS as first and second output optical beams,respectively.

Additional steps for optional use in conjunction with the disclosedsteps above, may be disclosed below. Step 267 may include: combining thefirst output optical beam and the second output optical beam to therebyform a combined optical beam, e.g., by beam combiner 302. Step 269 mayinclude: detecting the combined optical beam, e.g., by photodetector304. The method may also include the sub-step comprising modulating atleast one of: phase and amplitude of one or both of the first and secondoutput optical beams prior to combining the same in step 267 or aftercombining in step 267 and prior to step 269. Step 271 may include:transmitting the RF reference signal, e.g., by antenna 310.

In some embodiments, step 263 may further include passing a singleharmonic of the modulated beam and suppressing the remaining harmonicsof the modulated beam. Furthermore, the first and second output opticalbeams may be mutually coherent.

As noted, tunable laser 201 acts as the primary or reference laser usedto generate a seed signal, which injection-seeds the fixed laser 215.Use of a fixed laser 215 or other laser operating in a known or selected(e.g., programmable) range of frequencies allows for the inclusion of anoptical filter 213 with a narrow passband, which may be narrow enough toencompass only a single harmonic sideband from the modulation-sidebandcomb. Thus, the passband of the optical filter and the frequency of thefixed laser 215 may be matched. For example, the passband of the opticalfilter may be matched to the resonant frequency of the fixed laser 215and thus the passband of the optical filter may encompass the frequencyof the beam generated by the fixed laser 215. When the optical filter isfixed, a simpler more robust system may result.

Optical beams originally generated by laser beams of the tunable laser201 and the fixed laser 215 are then combined to form a combined beamhaving a beat frequency corresponding to the difference in frequenciesof the beams output by tunable laser 201 and fixed laser 215, such beatfrequency being an RF frequency corresponding to the RF referenceoscillator (e.g., an integer multiple thereof, which should beunderstood to include the integer of 1). As tunable laser 201 is tunableand the frequency of fixed laser 215 may be fixed, adjusting thefrequency of tunable laser 201 results in changing this difference inlaser beam frequencies and thus changing the RF beat frequency formed bythe combined beam. Specifically, adjusting the frequency of tunablelaser 201 shifts the comb of harmonics so that the optical filterselects a different harmonic (although within the same passbandfrequency). Continuous tuning of the RF beat frequency may be achievedby concurrent adjusting the frequency of tunable laser 201 and theRF-reference-oscillator-203 frequency Ω so as to maintain one of theteeth in the comb of harmonics within the passband of filter 213 andwithin the locking range of fixed laser 215.

These modifications may provide improved harmonic spur suppression. Forexample, filtering the sideband comb output by electro-optic modulator211 may be performed with a fixed optical filter. A fixed optical filtermay thus provide a narrow and more precise passband as compared to thetunable filter. For example, Fiber Bragg Grating (FBG) filters may beimplemented with passbands of 50 pm (6 GHz) wide. Such a narrow filtermay thus be used to select a single harmonic for injection locking priorto injection to laser 215, thereby relieving the injected laser cavity(of laser 215) of its filtering role. Thus, tuning of the laser 215 toselect one of several injected harmonics may be avoided, and the laser215 may be implemented as a fixed laser.

The significant reduction of spurs in the output of this modified TOPSconfiguration can be seen in FIG. 4, illustrating the spectrum of thebeat signal between the seed-injected laser 215 and the tunable laser201 when implemented according to the inventive concepts describedherein. As illustrated, the spurs of FIG. 4 are seen to be more than 40dB below the output tone, close to the noise floor of the spectrumanalyzer.

Additionally, the embodiment of FIG. 2 is capable of locking the laserbeams produced by the tunable laser 201 and fixed laser 215 usingmodulation-sideband injection locking. Thus, the beams produced by thetunable laser 201 and fixed laser 215 are mutually coherent, such mutualcoherence being maintained when combined and transmitted (e.g., in awave guide/optical fiber) by combiner 302. Therefore, when the combinedlight beam is impinged on photodetector 304, e.g., a photodiode, theoptical contributions to the phase noise (e.g., originating in the lasersources 201, 215) cancel each other out and a high quality RF signalthat has a frequency of the beat frequency corresponding to an integermultiple of the RF reference oscillator is generated (see RF output inFIG. 2). As spurs in the second output optical signal generated by theseed injected laser (e.g., laser 215) are significantlysuppressed/substantially eliminated, corresponding spurious beatfrequencies in the combined optical beam and corresponding spurious RFfrequencies in the RF signal produced by the photodetector 304 aresimilarly suppressed/avoided.

The foregoing is illustrative of exemplary embodiments and is not to beconstrued as limiting thereof. Although a few exemplary embodiments havebeen described, those skilled in the art will readily appreciate thatmany modifications are possible without materially departing from thenovel teachings and advantages of the inventive concepts. Accordingly,all such modifications are intended to be included within the scope ofthe present invention as defined in the claims.

What is claimed is:
 1. A tunable optical pair source (TOPS) apparatusconfigured to generate first and second output optical beams havingrespective first and second frequencies that are phase locked with eachother, the TOPS apparatus comprising: a tunable first laser configuredto generate a first laser beam at the first frequency, the firstfrequency being adjustable in response to tuning of the tunable firstlaser; a radio frequency (RF) oscillator configured to provide an RFreference signal oscillating at an RF frequency; a beam splitter inoptical communication with the tunable first laser and configured tosplit the first laser beam and transmit corresponding first and secondsplit beams; a first optical path in optical communication with the beamsplitter to receive the first split beam and to provide the first outputoptical beam to a first output; a second optical path in opticalcommunication with the beam splitter to receive the second split beamand to provide the second output optical beam to a second output, thesecond optical path comprising: an electro-optic modulator configured tomodulate the second split beam in response to the RF reference signal toform a modulated beam having a first sideband; an optical filterconfigured to receive the modulated beam and output a filtered opticalbeam; and a second laser configured to generate a second laser beam atthe second frequency, the second laser being configured to receive thefiltered optical beam as a seed, wherein the first frequency and thesecond frequency are offset by an integer multiple of the RF frequency,and wherein the second frequency falls within a passband of the opticalfilter.
 2. The TOPS apparatus of claim 1, further comprising: an opticalcirculator having first, second, and third ports; and wherein theoptical circulator is configured to receive the filtered optical beam atthe first port and pass the filtered optical beam via the second port tothe second laser, and receive the second laser beam at the second portand pass the second laser beam via the third port to the second output.3. The TOPS apparatus of claim 1, further comprising a combiner tocombine the first output optical beam and the second output optical beamto thereby form a combined optical beam.
 4. The TOPS apparatus of claim3, further comprising a photodetector configured to detect the combinedoptical beam.
 5. The TOPS apparatus of claim 4, further comprising avector modulator configured to receive the combined optical beam andmodulate at least one of: phase and amplitude of at least one of thefirst output optical beam and the second output optical beam of thecombined optical beam.
 6. The TOPS apparatus of claim 1, furthercomprising an amplifier configured to receive the RF reference signaland generate a RF comb of a plurality of RF harmonics, wherein theelectro-optic modulator is configured to modulate the second split beamwith the RF comb.
 7. The TOPS apparatus of claim 1, wherein the firstsideband comprise a first sideband comb comprising a plurality ofharmonics, wherein the optical filter has a passband of a width thatencompasses only a single harmonic of the first sideband comb of themodulated beam so as to attenuate the remaining harmonics of the firstsideband comb of the modulated beam.
 8. The TOPS apparatus of claim 7,wherein the remaining harmonics as provided with the second laser beamare no more than −20 dBc.
 9. The TOPS apparatus of claim 7, wherein thesecond laser is an injection-locked laser, and wherein the opticalfilter provides the filtered optical beam to the second laser and thesecond laser is configured to automatically lock to the single harmonicwithout tuning.
 10. The TOPS apparatus of claim 9, wherein the opticalfilter is a fixed optical filter having a passband matching anoperational frequency of the second laser.
 11. The TOPS apparatus ofclaim 7, wherein the first frequency of the tunable first laser isadjustable at least by an amount corresponding to a bandwidth of thefirst sideband comb.
 12. The TOPS apparatus of claim 7, wherein thetunable first laser is configured to adjust the first frequency of thefirst laser beam so that the passband of the optical filter encompassesa second harmonic of the modulated beam that is different than thesingle harmonic.
 13. The TOPS apparatus of claim 1, wherein a width ofthe passband of the optical filter is less than two times the RFfrequency of the RF reference signal.
 14. The TOPS apparatus of claim 1,wherein a width of the passband of the optical filter is 6 GHz or less.15. The TOPS apparatus of claim 13, wherein the optical filter is aFiber Bragg Grating filter.
 16. The TOPS apparatus of claim 1, furthercomprising at least one additional electro-optic modulator configured tomodulate at least one of: phase, amplitude, and frequency of the firstoutput optical beam and/or the second output optical beam.
 17. The TOPSapparatus of claim 3, wherein the combined optical beam has a beatfrequency of the integer multiple of the RF frequency.
 18. The TOPSapparatus of claim 17, further comprising: a photodetector configured todetect the combined optical beam and generate an electrical signalhaving a frequency of the integer multiple of the RF frequency; and anantenna in electrical communication with the photodetector to emit anelectromagnetic wave at a frequency of the integer multiple of the RFfrequency.
 19. The TOPS apparatus of claim 18, further comprising atleast one additional electro-optic modulator configured modulate atleast one of: phase, amplitude, and frequency of the first outputoptical beam and/or the second output optical beam to encodeinformation, wherein the encoded information is provided with theelectromagnetic wave emitted by the antenna.
 20. The TOPS apparatus ofclaim 1, wherein the first and second output optical beams are mutuallycoherent.
 21. The TOPS apparatus of claim 1, wherein phase noise of thesecond laser correlates to phase noise of the tunable first laser. 22.The TOPS apparatus of claim 1, wherein the optical filter is tunable.23. A tunable optical pair source (TOPS) apparatus configured togenerate first and second output optical beams having respective firstand second frequencies that are phase locked with each other, the TOPSapparatus comprising: a tunable first laser configured to generate afirst laser beam at the first frequency, the first frequency beingadjustable in response to tuning of the tunable first laser; a radiofrequency (RF) oscillator configured to generate an RF reference signaloscillating at an RF frequency; a beam splitter in optical communicationwith the tunable first laser and configured to split the first laserbeam and transmit corresponding first and second split beams; a firstoptical path in optical communication with the beam splitter to receivethe first split beam and to provide the first output optical beam to afirst output; a second optical path in optical communication with thebeam splitter to receive the second split beam and to provide the secondoutput optical beam to a second output, the second optical pathcomprising: an electro-optic modulator configured to modulate the secondsplit beam in response to the RF reference signal to form a modulatedbeam having a first sideband comprising at least one harmonic; a fixedoptical filter configured to receive the modulated beam and output theharmonic of the first sideband as a filtered optical beam; and a secondlaser configured to generate a second laser beam at the secondfrequency, the second laser being configured to receive the harmonic ofthe filtered optical beam as a seed; and a controller in communicationwith the tunable first laser and the RF oscillator, the controllerconfigured to alter the first frequency of the first laser generated bythe tunable first later and to alter the RF frequency of the RFreference signal generated by the RF oscillator; wherein the firstfrequency and the second frequency are offset by an integer multiple ofthe RF frequency, and wherein the second frequency falls within apassband of the optical filter.
 24. The TOPS apparatus of claim 23,wherein the controller is configured to alter the first frequency of thefirst laser generated by the tunable first later and to alter the RFfrequency of the RF reference signal generated by the RF oscillator in acoordinated manner so that the frequency of the harmonic of the firstsideband remains within the passband of the optical filter.
 26. The TOPSapparatus of claim 25, wherein the frequency of the harmonic is thesecond frequency.
 25. The TOPS apparatus of claim 23, wherein thecontroller is configured to alter the first frequency of the first lasergenerated by the tunable first later and to alter the RF frequency ofthe RF reference signal generated by the RF oscillator in a coordinatedmanner to set the frequency of the harmonic of the first sideband to thecenter frequency of the passband of the optical filter.
 26. The TOPSapparatus of claim 23, further comprising a combiner to combine thefirst output optical beam and the second output optical beam to therebyform a combined optical beam, wherein the combined optical beam has abeat frequency of an integer multiple of the RF frequency, and whereinthe controller is configured to alter the first frequency of the firstlaser generated by the tunable first laser and to alter the RF frequencyof the RF reference signal generated by the RF oscillator in acoordinated manner to alter the RF beat frequency of the combined signalwhile maintaining the frequency of the harmonic of the first sidebandwithin the passband of the optical filter.
 27. The TOPS apparatus ofclaim 26, wherein the controller is configured to alter the RF beatfrequency of the combined signal in real time.
 28. The TOPS apparatus ofclaim 23, wherein the harmonic is the Nth harmonic of a plurality ofharmonics of the first sideband, and the controller is configured tomaintain the Nth harmonic within the passband of the optical filterwhile altering the first frequency of the first laser generated by thetunable first later and the RF frequency of the RF reference signalgenerated by the RF oscillator in a coordinated manner.
 29. The TOPSapparatus of claim 28, wherein the passband of the optical filter is nomore than 6 GHz.
 30. The TOPS apparatus of claim 23, wherein theharmonic is the Nth harmonic of a plurality of harmonics of the firstsideband where N is an integer, and wherein, when the first frequency ofthe first laser generated by the tunable first laser is altered by Δω₁,the controller is configured to alter the RF frequency of the RFreference signal generated by the RF oscillator by 1/N*Δω₁.
 31. The TOPSapparatus of claim 30, wherein the controller is configured to alter thefirst frequency of the first laser and to alter the RF frequency of theRF reference signal in real time.
 32. A method of generating first andsecond output optical beams having respective first and secondfrequencies that are phase locked with each other, the methodcomprising: with a first tunable laser, generating a first laser beam ata first frequency; generating an RF reference signal having an RFfrequency; splitting the first laser beam and to provide correspondingfirst and second split beams; transmitting along a first optical paththe first split beam as the first output optical beam; transmitting thesecond split beam along a second optical path to generate the secondoutput optical beam, generating the second output optical beamcomprising: modulating the second split beam in response to the RFreference signal to form a modulated beam having a first sideband;filtering the modulated beam to provide a filtered optical beam; andusing the filtered optical beam as a seed for a second seed injectedlaser, generating a second laser beam at the second frequency, thesecond frequency falling within a passband of the optical filter andbeing offset from the first frequency by an integer multiple of the RFfrequency.